Chlorine/chloramine should be handled with water conditioner at each water change, there shouldn't be any chlorine at all.
You fish mix is (was) pretty wrong too...Betta should not be with any tankmates since they are aggressive, especially towards brightly coloured fish
60 liters sounds way too small for the number/type of fish as well, please clarify your numbers of each, your most recent water chemistry tests, you pH and whether you fully cycled the aquarium
The Betta appears to be rotting.....so we need your water chemistry numbers, fish numbers and cycle information before you go any further please